Non-aging steel



United States Patent NON-AGIN G STEEL No Drawing. .Application May 26," 1955, Serial No.-511,424 I 8 Claims. (Cl. 7sf'F-iz4 I This invention relates to a non-aging steel and a method of manufacture therefor and is more specificallyconcerned with a low-carbon, non-aging steel suitable for rolling into light gauge strip and sheet. v

' A great many products are made by cold drawing or otherwise deforming light gauge sheet steel. Steel suitable for such cold forming operations must be low in carbon content and also in other metalloids. Typically such steel contains carbon in amounts not greater than about .10% by weight, manganese between about .15 and 35%, phosphorus not greater than about .03 and sulfur not greater than about .04%. No alloying elements are usually added to such steel in the course of its manufacture although traces of other elements may be found therein, such elements being known as residuals and considered incidental to the conventional manufacture of steel using purchased scrap in the charge. The term low-carbon steel as used in this application will be understood to refer to steel within the composition range above mentioned.

Low-carbon steel strip and sheets for deep drawing and other coldforrning operations are usually produced by hot rolling slabs to hot band of an intermediate gauge, in the neighborhood of one-quarter inch, followed by cold rolling to the desired thickness. Theicold rolled sheet or strip is usually annealed to relieve the stresses built up in cold rolling and then given a light reduction in a cold rolling mill to flatten the material and provide it with a desirable surface, This skin or temper rolling, so-called, produces a reduction of only a few percent, seometimes less thanone percent. If the steel so treated is immediately cold drawn or otherwise for-med, optimum i'esults'are obtained. 'If the steel, however, is not used for a period of time, which maybe only a matter of weeks, it is found that deep drawing performance is generally considerably below the optimum. Mechanical tests made upon steel after such aging show that its ultimate tensile strength, yield point and hardness have increased, while its ductility as measured by elongation has decreased. Thisphenomenon as a wholeis known as aging and steel exhibiting it is known as aging steel.

' It has been known for many years'that the additiOn of certainelements to low-carbon steel will reduce or inhibit "ice have not changed greatly with the passage of time, but it will be observed that they are not as favorable to cold this aging. The following table compares the properties of an laging and a non-aging steel: V i

TABLE I.

Ultimate Yield Elonga Hardness Tensile Point tion (Rock- Strength (p. s. i.)' (percent) well B) .(p, I Steel A: Annealed and Skin l I Rolled -i 41, 400 21, 100 '41 33 SteelA; Aged Days 43, 30, 000 V 36 43 Steel B: Annealed and Skin v Y Rolled 42,100 17,400 40 38 Steel B: Aged 90 Days 42, 200 22, 300 I 40 40 The properties of rion aging Steel shown Table I aged condition.

' known aging steels in the non-aged condition.

drawing and forming as were those of Steel A in its nonaged condition. Steel B is harder than Steel A and exhibits slightly less elongation. This condition is generally true for conventional non-aging steels; their properties do not vary significantly with the passage of time but generally they are inferior to aging steels in a non- It will be understood by those skilled in the art that the term yield point as applied to non aging steels has a significance different from that attaching to aging steels, for fully non-aging steels have no yield point, strictly speaking, and the presence of a welldefined yield point is, in fact, an index of aging behavior in steel. Y

Non-aging steels are conventionally made by adding metallic aluminum to the steel either in the ladle or in the ingot mold, or both. Aluminum is, of course, a deoxidizer and when added in amounts of about two pounds per ton will fully deoxidize low-carbon steels so that such steels lie dead in the molds and pipe as they solidify. An aluminum addition of this magnitude is not sufficient, however, to render such steels non-aging. It is generally found that low-carbon steels must be treated with aluminum in amountsbetween about four and five pounds per ton to render them non-aging and the addition of this much aluminum is deleterious to the surface of the steel. The deep pipe which forms when such steels solidify can be partially counteracted by conventional hot topping practices, which are labor-consuming, or can be eliminated by a heavy top discard which, of course, is wastefuli All 'these factors combine to render non-aging steel made with heavy aluminum additions troublesome to produce.

Boron is known to be a deoxidizer, and will deoxidize low-carbon steels when added in amounts of about one pound per ton. Non-aging steels may be made by adding boron in amounts exceeding that figure but the presence of boron sufiicient to render the steel non-aging may render it hot-short at normal rolling temperatures, that is, susceptible to tearing and cracking upon hot rolling. For these reasons non-aging steels are not commercially made with boron additions.

It is an object of my invention therefore to provide a non-aging steel suitable for deep drawing which does not possess the disadvantages of aluminum or boron nonaging steels previously known. It is another object of my invention to provide a non-aging steel having mechanical properties comparable to those of previously It is another .object of my invention to provide a process of making non-aging steels suitable for deep drawing employing aluminum and boron in combination. It is another object of my invention-to provide a process of making a non-aging steel containing boron and aluminum which does not develop pipe on solidification. Other objects of my invention will appear in-the course of the following description thereof. I Y

I have found that when aluminum and boron are added together to a melt of low-carbon steel in proportions to be described, a non-aging steel results which has a number of desirable properties. I find that a non-aging steel can be made by adding aluminum in amount considerably less than required to deoxidize steel fully if added alone for this purpose, together iwith boron, also in amount less than would be required if boron were used alone for this purpose, and the resulting product is superior in certain ways to non-aging steels previously known. Itis essential to' my invention that the aluminum and boron be i added together, or alternatively, that the aluminumaddition precede the boron addition. If the boron is added before any aluminum, the advantages of my invention cannot be achieved. Under certain conditions. a. small aluminum addition may be made after the main addition of aluminum and boron, as will appear. Reference hereinafter to aluminum-boron additions will be understood to comprehend additions of aluminum and boron made simultaneously or of aluminum first followed by boron.

In a present preferred practice of my invention a melt of low-carbon steel is formed in the usual manner in an open hearth furnace. When the desired carbon content is attained, the heat may be partially deoxidized or blocked in the furnace by the usual ferro-manganese addition. When the heat is tapped into the ladle, I add metallic aluminum in amounts betweenabout one-quarter pound per ton and about one and three-quarterspounds per ton; preferably about three-quarters of a pound per ton. At the same time orsubsequently, I add boron in amount notlessthan about one quarter pound per ton but less than one pound per ton, preferably about threequarterspoundper ton. Smaller additions of boron are associated with larger additions of aluminum and vice versa, as will be described hereinafter. The aluminum and boron addition may be adjusted so that the steel teemed into ingot molds will show a small evolution of gas, not sufiicient to rim the steel, but suflicient to prevent any substantial amount of pipe as the steel solidifies. The ingots so produced will display either flat or slightly convex tops. Steels that are neither fully rimmed nor fully killed are generally called semi-killed steels, and this term may be applied to certain steels of my invention.

Steel ingots produced in accordance with my invention may be rolled into slabs and hot rolled into strip by conventional practices and do not display the tendency to tear or crack which is indicative of hot-shortness and is common to steels containing heavier boron additions. Hot rolled steel of my invention is non-aging as it comes from the run-out table even though it may be quenched in Water. In this respect it is superior to conventional non-aging steels made with aluminum additions only. Such steels are non-aging if allowed to cool normally in air after hot rolling but are rendered aging if they are water quenched after hot rolling. Conventional steels in this state require an annealing treatment to regain their non-aging properties. A preferred practice of my invention, therefore, includes the steps of hot rolling to hot band gauge steel made as described herein, water quenching it, and then cold rolling it to final gauge without any previous annealing.

It is well known that the efiiciency of many elements added to steel in the ladle and mold is low, that is, the r amount of such element found by chemical analysis in the steel itself is less than that which would be expected from the known amount added. This is true of the aluminum and boron added to the steel of. my invention.

I find that when aluminum and boron are added in amounts of about three-quarters pound per ton each the boron contents of the steel will generally range between .006% and about .01576. The aluminum content will usually not drop below 005% and not exceed about 015%. I find that the boron content of steels of my invention should not be less than about .006% if non-aging properties are to be obtained. Boron contents up to about .020% are not deleterious, but above this figure lead to an increasing tendency to the hot-shortness previously mentioned. It should be understood that by careful regulation of rolling temperatures moderately hot-short steels can be rolled with success. If the boron content of my steels is held below the previously mentioned figure of .015 however, no special rolling precautions will usually be found necessary. I find that a totalboron and aluminum content of between about .0ll% and 025% is generally satisfactory.

Table II lists the physical properties of typical steels of my invention.

Both steels C and D are seen to be non-aging. Steel' C is seen to exhibit hardness-camp'arable to that of'Steel A of Table I in its non-aged condition. Both steels C and D after aging exhibit elongations only slightly less than that of Steel A before aging.

As I have indicated, the relative amounts of aluminum and boron. required for the steels of my inventionare adjusted with respect to" each other. Stated generally, if either element is added in amount not less than about three-quarters of a pound per ton but less thanithe' amount required to deoxidize the steel fully, the other element should be. added in amount not less than about threequarters of a pound per ton but less than about one pound per ton. Steel so made will always. be non-aging but not necessarily semi-killed as indicated by absence of pipe in the ingot. Since boron is a relatively expensive addition, it is economic to reduce the required amount as far as possible, but'I find it cannot be reduced below one-quarter pound per ton even though it is accompanied by a much larger aluminum addition. A larger aluminum addition improves the efficiency of the boron so that the previously mentioned desirable minimum boron content of about 006% in the steel can be obtained even with smaller boron additions within. the range above mentioned.

The production of semi-killed ingots of non-aging, lowcarbon'steel is a matter of careful? control of deoxidizer addition. If too little deoxidizer is added'for the dissolved oxygen in the steel, the latter willlrim in the ingot molds, and will not possess non-aging characteristics. If too muchv is. added, the steel will pipe upon solidfication.

It is not easy to gauge the oxygen content of the steel in the furnace closely and, furthermore, the action of the steel in the molds is influenced by other factors, such as the temperature of the ingot molds and even the tem-l perature of the surrounding air. Because ofthesevari-l ables, the proper amounts of aluminum and boron to be added to a melt of low-carbon steel to produce semikilled ingots can. be stated only with a fair degree of approximation, as those skilled in the art will appreciate. A small boron addition requires a large aluminum addi tion' and vice versa. Within the limits I have set out for boron additions, the amounts of aluminum and boron which will confer non-aging properties on low-carbon steel without deoxidizing it beyond a semi-killed state may be determined approximately fromthe empirical relation Aluminum 2 X Boron=2 A Where the units represent pounds of deoxidizing element per ton of steel. In accordance with this relation, a boron addition of one-quarter pound per ton requires one: and three-quarter pounds per ton of aluminum, while a. boronaddition just under one pound per ton requires a little more than one-quarter pound per ton of aluminum. I

I find it desirable when making semi-killed ingots with minimum boron contents, to add the bulk of the aluminum to the, steel before or along with the boron addition, and add the remainder of the aluminum to the steel in the'molds. In this way the "mold action of the steel can be more readily controlled. Not more than about onequarter pound per ton of aluminum, however, should be' added to the molds in this way.

I claim:

1-. A method of making non-aging steel comprising preparing a melt of low-carbon steel, adding thereto aluminum in amount not less than three-quarters of a pound per ton but less than that amount required to deoxidize said steel fully and boron in amount not less than threequarters of a pound per ton but less than one pound per ton.

2. A method of making a non-aging steel comprising preparing a melt of low-carbon steel, adding thereto aluminum in amount between about three-quarters of a pound per ton and one pound per ton and boron in amount not less that about three-quarters of a pound per ton but less than that amount required to deoxidize said steel fully.

3. A method of making a semi-killed, non-aging steel comprising preparing a melt of low-carbon steel and adding thereto boron in amount not less than one-quarter pound per ton but less than one pound per ton and aluminum, the relative amounts of boron and aluminum in pounds per ton being determined from the relation Aluminum 2 X Boron=2 A 4. A method of making a semi-killed, non-aging steel comprising preparing a melt of low-carbon steel and adding thereto boron in amount not less than about onequarter pound per ten but less than about one pound per ton, and aluminum, the relative amounts of boron and aluminum in pounds per ton being determined from the relation Aluminum 2 X Boron=2 A not more than about one-quarter pound per ton of aluminum being added to said steel in the ingot molds.

5. A method of making semi-killed non-aging steel strip comprising preparing a melt of low-carbon steel, adding thereto boron in amount not less than one-quarter pound per ton but less than one pound per ton, and aluminum, the relative amounts of boron and aluminum in pounds per ton being determined from the relation Aluminum 2 X Boron=2 A hot rolling said steel to hot band gauge, water quenching it, and cold rolling it to final gauge without previous annealing.

6. A non-aging steel containing carbon less than .10% by weight, manganese between .15% and phosphorus .03% maximum, sulfur .04% maximum, boron between about .006 and .020% and aluminum between about .005 and .015 the balance being iron and incidental impurities.

7. A non-aging steel containing carbon less than .10% by weight, manganese between .15% and 35%, phosphorus .03% maximum, sulfur .04% maximum, boron not less than about .006% and aluminum not less than about 005%, the balance being iron and incidental impurities, the total boron and aluminum content being not greater than about .025%.

8. A non-aging steel containing carbon less than .10% by weight, manganese between .15% and 35%, phosphorus .03% maximum, sulfur .04% maximum, boron between about .006 and .015 and aluminum between about .005 and .015%, the balance being iron and incidental impurities,

References Cited in the file of this patent The Iron Age, June 16, 1949, page 90. 

1. A METHOD OF MAKING NON-AGING STEEL COMPRISING PREPARING A MELT OF LOW-CARBON STEEL, ADDING THERETO ALUMINUM IN AMOUNT NOT LESS THAN THREE-QUARTERS OF A POUND PER TON BUT LESS THAN THAT AMOUNT REQUIRED TO DEOXIDIZE SAID STEEL FULLY AND BORON IN AMOUNT NOT LESS THAN THREEQUARTERS OF A POUND PER TON BUT LESS THAN ONE POUND PER TON. 